(1.) Field of the Invention
The present invention relates to a secondary battery having a high energy density, a high charging-discharging efficiency, a long cycle life, a low self-discharge rate, and a good voltage levelness at the time of discharging.
(2.) Description of the Prior Art
It is known that an acetylene high polymer prepared by polymerizing acetylene by using a so-called Ziegler-Natta catalyst comprising a transition metal compound and an organometallic compound exhibits an electrical conductivity falling within the semiconductor region and is therefore useful as an electrical or electronic element material.
As means for preparing practical shaped articles of acetylene high polymers, there are known the following methods.
(a) A method in which a powdery acetylene high polymer is compression-molded. PA0 (b) A method in which under specific polymerization conditions, polymerization is carried out to prepare a film of an acetylene high polymer having a fibrous microcrystalline (fibril) structure and having a high mechanical strength (see Japanese Examined Patent Publication No. 32581/73).
It also is known that when a shaped article of the powdery acetylene high polymer prepared according to the method (a) is chemically treated with an electron-accepting compound (electron acceptor) such as BF.sub.3, BCl.sub.3, HCl, Cl.sub.2, SO.sub.2, NO.sub.2, HCN, O.sub.2 or NO, the electrical conductivity is increased about 1,000 times at the highest, and in contrast, when the shaped article is treated with an electron-donating compound (electron donor) such as ammonia or methylamine, the electrical conductivity is reduced to about 1/10,000 in an extreme case.
Furthermore, it is known that when the filmy acetylene high polymer prepared according to the method (b) is chemically doped with an electron-accepting compound such as I.sub.2 , Cl.sub.2, Br.sub.2, ICl, IBr, AsF.sub.5, SbF.sub.5 or PF.sub.6, or an electron-donating compound such as Na, K or Li, the electrical conductivity of the acetylene high polymer can be freely controlled within a broad range of from 10.sup.-8 to 10.sup.+3 .OMEGA..sup.-1 .multidot.cm.sup.-1. It has been proposed that this doped filmy acetylene high polymer be used as a material of a positive electrode of a primary battery.
In addition to the above-mentioned chemical doping method, there has been developed a method in which a p-type or n-type electrically conductive acetylene high polymer is prepared by electrochemically doping an acetylene high polymer with an anion such as ClO.sub.4.sup.-, PF.sub.6.sup.-, AsF.sub.6.sup.-, AsF.sub.4.sup.-, CF.sub.3 SO.sub.3.sup.- or BF.sub.4.sup.-, or a cation such as R'.sub.4 N.sup.+ (in which four R' groups are the same alkyl groups). A re-chargeable battery comprising an acetylene high polymer film prepared according to the method (b), which is electrochemically doped, has been reported. This battery comprises, as positive and negative electrodes two acetylene high polymer films having a thickness of, for example, 0.1 mm, which are obtained according to the method (b). When this battery is immersed in a tetrahydrofuran solution containing lithium iodide and is connected to a 9-V direct current power source, the lithium iodide is electrolyzed, the acetylene high polymer film as the positive electrode is doped with iodine and the acetylene high polymer film as the negative electrode is doped with lithium. This electrolytic doping corresponds to the charging step. If a load is connected to the two doped electrodes, the lithium ion is reacted with the iodine ion and an electric power can be taken out. In this case, the open circuit voltage (Voc) is 2.8 V and the short circuit current density is 5 mA/cm.sup.2. When a tetrahydrofuran solution containing lithium perchlorate is used as the electrolyte, the open circuit voltage is 2.5 V and the short circuit current density is about 3 mA/cm.sup.2.
Two acetylene high polymer films having a thickness of 0.1 mm, which are prepared according to the method (b), are independently wrapped with platinum meshes having a lead line taken out therefrom. When these wrapped films are immersed in an acetonitrile solution containing 1 mole/l of tetrabutylammonium perchlorate and charging is conducted with a constant current of 5 mA/cm.sup.2 for a certain time, the acetylene high polymer film as the negative electrode is doped with the tetrabutylammonium ion and the acetylene high polymer film as the positive electrode is doped with the perchlorate ion. In this case, the open circuit voltage (Voc) of the battery is 2.5 V. If this battery is discharged at 1 mA/cm.sup.2 until the battery voltage is reduced to 1.0 V, the discharge electricity can be taken out in a quantity corresponding to 81% of the quantity of the charge electricity.
Since these known batteries are formed by using as the electrode material an acetylene high polymer which is capable of providing a light-weight and small-size battery, these batteries have attracted attentions as cheap batteries having a high energy density, the weight and size of which can be easily diminished.
Almost all of electrolytes used as dopants in the above-mentioned known techniques have a low solubility in a solvent having a relatively broad range of stable voltages or they give electrolytic solutions having a low electrical conductivity, or the electrolytes per se or electrolysis products thereof have a reactivity with a solvent having a relatively broad range of stable voltages. Therefore, almost all of the known electrolytes cannot be used for solvents having a relatively broad stable voltage range.
For example, metallic lithium has a reactivity with a nitrile type solvent having a relatively broad stable voltage range, and therefore, use of a lithium salt comprising metallic lithium as the cation component as an electrolyte is not recommended. Furthermore, a tetrabutylammonium salt such as mentioned above is relatively highly soluble in a nitrile type solvent, and if the tetrabutylammonium salt is used as the electrolyte, a high charging-discharging efficiency can be obtained, but a satisfactory energy density cannot be obtained.
When a tetraethylammonium salt, which is another alkyl ammonium salt, is used as the electrolyte, the solubility of the tetraethylammonium salt in benzonitrile having a broad stable voltage range is very low and the saturation solubility at room temperature is lower than 1 mole/l. Accordingly, an electrolytic solution formed by dissolving the tetraethylammonium salt in benzonitrile is disadvantageous from the viewpoint of the energy density, and this solution cannot be used as an electrolytic solution for a battery having a high energy density.
Accordingly, development of an electrolyte, that is, a dopant, having a high solubility in an organic solvent having a relatively broad stable voltage range and a molar molecular weight as low as possible, being capable of giving a high electrical conductivity and having a good electrochemical stability and a low reactivity with a polymeric compound having conjugated double bonds in the main chain, which is used as the electrode, has been eagerly desired.